The present invention relates to an electronic ignition device for use, in particular, with the internal combustion engines of motor vehicles. This device comprises, in association with a device of the conventional type enabling ignition to be advanced as a function of the speed of rotation of the engine, a circuit by means of which ignition can be retarded as required at any stage of operation, i.e. whatever the speed of rotation of the engine. In other words, the electronic ignition device enables ignition to be retarded by the application of an electrical signal, retardation being superposed upon the conventional ignition advance at any point along the advance curve.
It is in fact known that, to achieve optimum performance in internal combustion engines, it may be necessary to take into account a large number of operational parameters such as, for example, outside temperature, temperature of the exhaust gases, temperature of the cooling water, the occurrence of "pinking", the level of the vacuum in the admission manifold, so that consequently, on the basis of the advance curve that enables a conventional advance device to be obtained, it may be felt desirable to modify the ignition advance angle. In general, monitoring of the parameters that have to be taken into account takes the form of an electrical signal, the value of which determines the extent of the correction to be made to the ignition advance. The purpose of the invention is to provide an electronic device which, on the basis of a voltage level constituting the signal, enables ignition retardation of a predetermined value to be obtained, that is to say ignition retardation which is neither a function of the speed of rotation of the engine at the moment of correction, nor a function of the manufacturing tolerances of the various elements that are used to form the ignition device. More particularly, the invention relates to an ignition device which uses a magnetic pick-up, the coil of which delivers a periodical alternating signal, ignition being started at the moment when the voltage supplied by the pick-up passes through zero in a zone of rapid change.
In the ignition devices of this type, which are well-known in the state of the art, use is generally made of a permanent magnet which generates a magnetic field, said magnet generating a magnetic flux in a magnetic circuit into which is introduced a cam rigidly connected to the ignition device, the fixed part of the magnetic circuit being separated from the above-mentioned cam by at least one air-gap; an electromagnetic coil surrounds a part of the magnetic circuit and receives at its terminals a voltage induced by the changes in the flux in the circuit. It is known that the induced voltage furnished by such a pick-up is proportional to the rate of change in the flux, so that the signal obtained has a form and amplitude which are a function of the speed of rotation of the spindle. It has already been proposed, in French Pat. No. 2.230.967, to associate, with such a pick-up, an integrator for obtaining a signal, the shape of which no longer depends upon speed of rotation of the spindle of the ignition device. A further purpose of the invention is to provide a means which uses a magnetic pick-up associated with such an integrator, as described in French Pat. No. 2.230.967.
It is clear that if the output signal from an integrator circuit associated with a magnetic pick-up as defined above is compared with a voltage threshold of predetermined value, it is possible to define an ignition displacement of predetermined amplitude independently of the speed of rotation of the ignition device and, therefore, of the engine. However, for a given comparison threshold, the ignition angle displacement will clearly be a function of the signal furnished by the pick-up and its associated integrator, that is to say the ignition angle displacement will not be capable of being precisely fixed, and this is very disadvantageous when the motor is required to function to give optimum performance, as well as when the aim is to reduce and to effect a saving in energy. Another object of the invention is to provide an electronic means whereby the ignition displacement which is achieved with, on the one hand, a magnetic pick-up associated with an integrator, and, on the other, a signal having a given voltage level, can be rendered independent of manufacturing tolerances.